1. Field of the Invention
This invention relates to focusing screens and, more particularly, to phase-type focusing screens suited to photographic cameras, and video cameras.
2. Description of the Related Art
There have been proposed focusing screens of the phase type in which on an optically transparent substrate a minute pattern comprised of a convex or concave portion is formed to a predetermined shape, a plurality of minute patterns are periodically arrayed in a unit region and a plurality of unit regions are regularly located in a rectangular form or triangular form.
For example, Japanese Laid-Open Patent Application No. Sho 55-9568 proposes a form of the phase-type focusing screen in which the minute patterns are round in shape and the same in size, and their array is regular and repetitive.
Besides this, in Japanese Laid-Open Patent Application No. Sho 62-269127, the minute patterns comprised of a convex or concave portion are regularly arrayed on the surface of the transparent substrate so that a phase difference is imparted to the passing light beam. Thus, the phase-type focusing screen has a desired diffusion characteristic.
The focusing screens proposed in these publications, because of the use of the relatively high periodicity in the array of the minute patterns, causes strong diffraction of light due to the wavelengths thereof. Therefore, chromatic blur appears when the image formed on the screen by the photographic system is blurred, and the diffracted light has an intensity distribution having peaks dispersively. The conventional phase-type focusing screens have difficulties in that blurring of the image projected thereon is lowered.